Nature has developed innovative ways to solve a sticky challenge. Mussels and barnacles stubbornly glue themselves to cliff faces, ship hulls, and even the skin of whales. Likewise, tendons and cartilage stick to bone with incredible robustness, giving animals flexibility and agility. The natural adhesive in all these cases is hydrogel, a sticky mix of water and gummy material that creates a tough and durable bond.
Engineers at MIT have developed a method to make synthetic, sticky hydrogel that is more than 90 percent water. The hydrogel, which is a transparent, rubber-like material, can adhere to surfaces such as glass, silicon, ceramics, aluminum, and titanium with a toughness comparable to the bond between tendon and cartilage on bone. In experiments to demonstrate its robustness, the researchers applied a small square of their hydrogel between two plates of glass, from which they then suspended a 55-pound weight. They also glued the hydrogel to a silicon wafer, which they then smashed with a hammer. While the silicon shattered, its pieces remained stuck in place. Such durability makes the hydrogel an ideal candidate for protective coatings on underwater surfaces such as boats and submarines. As the hydrogel is biocompatible, it may also be suitable for a range of health-related applications, such as biomedical coatings for catheters and sensors implanted in the body.
Lead study author Xuanhe Zhao and coworkers started out building their tough hydrogels the way other research teams have before. In each of their recipes, they used two different components. Each was a polymer, a substance made from repeating building blocks. One polymer had permanent cross-links and was highly stretchable. The other had reversible cross-links, so that it could dissipate mechanical energy when stretched or pulled, a way to get around brittleness and cracking. The team needed to create a tough interaction between something gooey and flexible (a hydrogel) and a rigid surface (glass, titanium, etc). Many research teams have tried to attain this very goal, but the most common result is an interaction that’s too brittle or weak to be broadly useful. The best results have often come from substances that don’t have as high a water content. Here’s what the MIT team did differently. Before adhering the hydrogel to rigid surfaces, they pre-treated the surfaces with a chemical anchor that was customized to stick only to the stretchable polymer component. The team reasoned this chemical anchoring would be enough to provide tough hydrogel-solid bonding
A tough, flexible hydrogel that bonds strongly requires two characteristics, Zhao found, energy dissipation and chemical anchorage. A hydrogel that dissipates energy is essentially able to stretch significantly without retaining all the energy used to stretch it. A chemically anchored hydrogel adheres to a surface by covalently bonding its polymer network to that surface. “Chemical anchorage plus bulk dissipation leads to tough bonding,” Zhao says. “Tendons and cartilage harness these, so we’re really learning this principle from nature.”
In developing the hydrogel, Yuk mixed a solution of water with a dissipative ingredient to create a stretchy, rubbery material. He then placed the hydrogel atop various surfaces, such as aluminum, ceramic, glass, and titanium, each modified with functional silanes molecules that created chemical links between each surface and its hydrogel. The researchers then tested the hydrogel’s bond using a standard peeling test, in which they measured the force required to peel the hydrogel from a surface. On average, they found the hydrogel’s bond was as tough as 1,000 joules per square meter about the same level as tendon and cartilage on bone. Zhao group compared these results with existing hydrogels, as well as elastomers, tissue adhesives, and nanoparticle gels, and found that the new hydrogel adhesive has both higher water content and a much stronger bonding ability. “We basically broke a world record in bonding toughness of hydrogels, and it was inspired by nature,” Yuk says
“You can imagine new applications with this very robust, adhesive, yet soft material,” says Xuanhe Zhao, the Robert N. Noyce Career Development Associate Professor in MIT’s Department of Mechanical Engineering. For example, Zhao’s group is currently exploring uses for the hydrogel in soft robotics, where the material may serve as synthetic tendon and cartilage, or in flexible joints. “It’s a pretty tough and adhesive gel that’s mostly water,” Hyunwoo Yuk, a graduate student in mechanical engineering and the lead author of a paper on the work, says. “Basically, it’s tough, bonding water.”
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